Preparation and Solution Behavior of a Thermoresponsive Diblock

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Preparation and Solution Behavior of a Thermoresponsive Diblock Copolymer of Poly(ethyl glycidyl ether) and Poly(ethylene oxide) Michihiro Ogura, Hiroyuki Tokuda, Shin-ichiro Imabayashi,*,† and Masayoshi Watanabe* Department of Chemistry and Biotechnology, Yokohama National UniVersity, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ReceiVed May 13, 2007. In Final Form: June 12, 2007 A thermoresponsive diblock copolymer, poly(ethyl glycidyl ether)-block-poly(ethylene oxide) (PEGE-b-PEO), is synthesized by successive anionic ring-opening polymerization of ethyl glycidyl ether and ethylene oxide using 2-phenoxyethanol as a starting material, and its solution behavior is elucidated in water. In a dilute 1 wt % solution, the temperature-dependent alteration in the polymer hydrodynamic radius (RH) is measured in the temperature range between 5 and 45 °C by pulse-gradient spin-echo NMR and dynamic light scattering. The RH value increased with temperature in two steps, where the first step at 15 °C corresponds to the core-shell micelle formation and the second step at 40 °C corresponds to the aggregation of the core-shell micelles. The formation of the core-shell micelles is supported by the solubilization of a dye (1,6-diphenyl-1,3,5-hexatriene) in the hydrophobic core, which is recognized for a copolymer solution in the temperature range between 20 and 40 °C. In this temperature range, the core-shell micelles and the unimers coexist and the fraction of the former gradually increases with increasing temperature, suggesting equilibrium between the micelles and the unimers. In the concentrated regime (40 wt % solution), the solution forms a gel and the small-angle X-ray scattering measurements reveal the successive formation of hexagonal and lamellar liquid crystal phases with increasing temperature.

1. Introduction The behavior of amphiphilic block copolymers in aqueous solutions has attracted considerable attention in recent decades because of their new applications as gene and drug delivery systems,1,2 microreactors for chemical synthesis and catalysis,3-5 polymeric surfactants for stabilization of colloid dispersions,6-8 and so forth. Self-association of these copolymers leads to the formation of various structures with dimensions ranging from nano- to microscopic.9-11 Similar to low-molecular-weight surfactants, micellization of amphiphilic block copolymers in nonsolvents for one of the segments produces nanoscale coreshell micelles, where the insoluble segment forms the core and the soluble segment forms the shell of the resultant core-shell micelles.12-19 The spherical aggregates may interact with each other to form networks or domain structures. One of the interesting amphiphilic block copolymers is a series of double-hydrophilic block copolymers, in which one of the hydrophilic blocks undergoes a transition from soluble to insoluble * To whom correspondence should be addressed. Telephone/Fax: +813-5859-8159 (S.I.); 81-45-339-3955 (M.W.). E-mail: [email protected] (S.I.); [email protected] (M.W.). † Present address: Department of Applied Chemistry, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan. (1) Yokoyama, M.; Kwon, G. S.; Okano, T.; Sakurai, Y.; Seto, T.; Kataoka, K. Bioconjugate Chem. 1992, 3, 295-301. (2) Jeong, B.; Bae, Y. H.; Lee, D. S.; Kim, S. W. Nature 1997, 388, 860-862. (3) Persigehl, P.; Jordan, R.; Nuyken, O. Macromolecules 2000, 33, 69776981. (4) Nishikawa, H.; Morita, T.; Sugiyama, J.; Kimura, S. J. Colloid Interface Sci. 2004, 280, 506-510. (5) Wang, Y.; Wei, G.; Zhang, W.; Jiang, X.; Zheng, P.; Shi, L.; Dong, A. J. Mol. Catal. A: Chem. 2007, 266, 233-238. (6) Bronstein, L.; Sidorov, S.; Valetsky, P.; Hartman, J.; Colfen, H.; Antonietti, M. Langmuir 1999, 15, 6256-6262. (7) Creutz, S.; Jerome, R. Langmuir 1999, 15, 7145-7156. (8) Zheng, P.; Jiang, X.; Zhang, X.; Zhang, W.; Shi, L. Langmuir 2006, 22, 9393-9396. (9) Soler-Illia, G. J. A. A.; Crepaldi, E. L.; Grosso, D.; Sanchez, C. Curr. Opinion Colloid Interface Sci. 2003, 8, 109-126. (10) Alexandridis, P.; Spontak, R. J. Curr. Opinion Colloid Interface Sci. 1999, 4, 130-139 and references included in this article. (11) Lazzarri, M.; Lo´pez-Quintela, M. A. AdV. Mater. 2003, 15, 1583-1594.

in an aqueous solution, collapses, and creates a hydrophobic microdomain in a manner analogous to those of polymer surfactants when passing through the critical temperature (Tc).20-32 Typical examples are poly(N-isopropylacrylamide)-b-poly(ethylene oxide) (PNIPA-b-PEO)24-30 and PEO-poly(propylene oxide)-PEO (PEO-PPO-PEO).31,32 The former copolymer selfassembles into a polymer micelle consisting of a PNIPA core and a hydrophilic shell of PEO at above 32 °C in water.24-30 We synthesized a new series of thermosensitive polyethers by anionic ring-opening polymerization of glycidyl ether derivatives. (12) Tao, J.; Stewart, S.; Liu, G.; Yang, M. Macromolecules 1997, 30, 27382745. (13) Borisov, O. V.; Zhulina, E. B. Macromolecules 2003, 36, 10029-10036. (14) Gohy, J.-F.; Varshney, S. K.; Je´roˆme, R. Macromolecules 2001, 34, 33613366. (15) Martin, T. J.; Procha´zka, K.; Munk, P.; Webber, S. E. Macromolecules 1996, 29, 6071-6073. (16) Stapert, H. R.; Nishiyama, N.; Jiang, D.; Aida, T.; Kataoka, K. Langmuir 2000, 16, 8182-8188. (17) Michels, B.; Waton, G.; Zana, R. Langmuir 1997, 13, 3111-3118. (18) Mortensen, K.; Skov, J. Macromolecules 1993, 26, 805-812. (19) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 41454159. (20) Arotcarena, M.; Heise, B.; Ishaya, S.; Laschewsky, A. J. Am. Chem. Soc. 2002, 124, 3787-3793. (21) Dimitrov, P.; Rangelov, S.; Dworak, A.; Tsvetanov, C. B. Macromolecules 2004, 37, 1000-1008. (22) Kim, M. S.; Hyun, H.; Khang, G.; Lee, H. B. Macromolecules 2006, 39, 3099-3102. (23) Choi, C.; Chae, S. Y.; Nah, J.-W. Polymer 2006, 47, 4571-4580. (24) Motokawa, R.; Morishita, K.; Koizumi, S.; Nakahira, T.; Annaka, M. Macromolecules 2005, 38, 5748-5760. (25) Zhang, W.; Shi, L.; Wu, K.; An, Y. Macromolecules 2005, 38, 57435747. (26) Virtanen, J.; Holappa, S.; Lemmetyinen, H.; Tenhu, H. Macromolecules 2002, 35, 4763-4769. (27) Topp, M. C. D.; Dijkstra, P. J.; Talsma, H.; Feijen, J. Macromolecules 1997, 30, 8518-8520. (28) Qin, S.; Geng, Y.; Discher, D. E.; Yang, S. AdV. Mater. 2006, 18, 29052909. (29) Wu, K.; Shi, L.; Zhang, W.; An, Y.; Zhu, X.-X. J. Appl. Polym. Sci. 2006, 102, 3144-3148. (30) Kwon, I. K.; Matsuda, T. Biomaterials 2006, 27, 986-995. (31) Zhou, Z.; Chu, B. J. Colloid Interface Sci. 1988, 126, 171-180. (32) Brown, W.; Schille´n, K.; Almgren, M.; Hvidt, S.; Bahadur, P. J. Phys. Chem. 1991, 95, 1850-1858.

10.1021/la701384q CCC: $37.00 © 2007 American Chemical Society Published on Web 08/03/2007

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The prepared polymers have controllable molecular weights with narrow molecular weight distributions, and they exhibit a coilglobule transition in water whose temperature can be changed from 15 to 58 °C by altering the side-chain units.33 The new polymers have a main-chain structure similar to that of the biocompatible PEO, and they are expected to be useful for biochemical and biomedical applications. We realized the thermal control of the biocatalytic activity of glucose oxidase using phenothiazine-labeled poly(ethoxyethyl glycidyl ether) as a mediator.34 We also reported that the redox properties of chainend phenothiazine (PT) for PT-labeled poly(ethyl glycidyl ether)block-poly(ethylene oxide) (PT-PEGE-b-PEO) change, accompanying the phase transition of the temperature-sensitive PEGE segment.35 The present paper reports the preparation of poly(ethyl glycidyl ether)-block-poly(ethylene oxide) (PEGE-b-PEO), which has a phenyl group at the chain end instead of PT, and its solution behavior in water. The dilute solution behavior of the copolymer is discussed based on a temperature-dependent change in the hydrodynamic radius (RH) of the polymer particles calculated from the self-diffusion coefficient (D) that was measured in the temperature range between 5 and 45 °C by pulse-gradient spinecho NMR (PGSE-NMR) and dynamic light scattering (DLS). The RH value increases with temperature in two steps: the first step corresponds to the core-shell micelle formation and the second step corresponds to the aggregation of the core-shell micelles. In concentrated aqueous conditions, small-angle X-ray scattering (SAXS) measurements reveal the formation of hexagonal and lamellar liquid crystal phases. 2. Experimental Section 2.1. Materials. Ethyl glycidyl ether (EGE) was purchased from Tokyo Kasei and purified by repeated distillation until no trace of epichlorohydrin was detected by gas chromatography. 2-Phenoxyethanol, ethylene oxide (EO), diethylene glycol dimethyl ether (diglyme), and all-trans-1,6-diphenyl-1,3,5-hexatriene (DPH) were purchased from Kanto Chemical, Mitsubishi Chemical, Junsei Chemical, and Aldrich, respectively. 2.2. Synthesis of PEGE and PEGE-b-PEO. 2-Phenoxyethanol (6.7 mmol) was dissolved in dried diglyme (80 mL), and then dried potassium hydroxide (3.3 mmol) was added as a catalyst. After this mixture was introduced into an autoclave, the content was stirred at 60 °C for 1 h to yield potassium phenoxyethoxide, followed by several repeated evacuation-N2 flow cycles at 40 °C to remove the water produced in the initiation reaction. An appropriate amount of EGE (187 mmol for the preparation of the polymer with the numberaverage molecular weight (Mn) of 3000) was introduced to the mixture under N2 gas flow, and the reaction mixture was stirred at 110 °C for 20 h under anhydrous conditions. After the complete consumption of the monomer, the pH of the reaction mixture was adjusted to 5-6 by adding 1 wt % sulfuric acid to terminate the polymerization. To remove ionic impurities, the mixture was stirred at 60 °C with an acid-adsorbent for 30 min and with a base-adsorbent for 1 h and then filtrated. The filtrate was evaporated to dryness at 100 °C for 1 h under reduced pressure, yielding PEGE as a light-yellow viscous liquid. For the preparation of PEGE-b-PEO, the appropriate amount of EO to obtain the desired molecular weight of the PEO segment was gradually introduced into the autoclave at 60 °C from a tank successively after the polymerization of EGE. The reaction mixture was stirred at 110 °C for 8 h, while the inside pressure of the autoclave was maintained at 2.0 × 105 Pa by N2. After the reaction mixture (33) Aoki, S.; Koide, A.; Imabayashi, S.; Watanabe, M. Chem. Lett. 2002, 1128-1129. (34) Nakadan, N.; Imabayashi, S.; Watanabe, M. Langmuir 2004, 20, 87868791. (35) Tsuda, R.; Kaino, S.; Kokubo, H.; Imabayashi, S.; Watanabe, M. Colloids Surf., B 2007, 56, 255-259.

Ogura et al. Table 1. Composition, Molecular Weight, Molecular Weight Distribution, and Yield of Thermoresponsive Polymers Used in This Work sample

no. of PEGE unitsa

PEGE3300 PEGE6600 PEGE8800 PEGE-b-PEO

31 63 85 55

a

no. of PEO unitsa

Mna

Mw/Mnb

yield (%)

93

3300 6600 8800 9900

1.22 1.29 1.25 1.26

95 95 93 80

Determined by 1H NMR or calculated using NMR data. b

Determined

by GPC.

was cooled down to room temperature, the remaining EO gas was quenched by passing it through a NaOH solution. Similar afterreaction treatments as those for PEGE yielded PEGE-b-PEO as a transparent viscous liquid. The Mn and the molecular weight distribution (Mw/Mn) of the obtained polymers were characterized by 1H NMR spectroscopy and gel permeation chromatography (GPC), respectively (Table 1). The numbers of the EGE and (EGE + EO) units were estimated from the ratio of the integrated signal intensities of the side-chain end methyl protons/phenolic protons and of the protons detected at 3.7 ppm/phenolic protons, respectively. All the polymers exhibit relatively narrow molecular weight distributions of the order of 1.2. For the PEGE homopolymers, the Mn value increases proportionally to the polymerization time of EGE up to the value determined by the feed ratio of the monomer to phenoxyethanol, indicating a livinglike character of the anionic polymerization. For the copolymer, on the other hand, the numbers of the EGE and EO units (55 and 93) for the obtained copolymer are slightly different from those (48 and 110) expected from the feed ratio and monomer conversion. This suggests that a small amount of starting materials did not participate in the polymerization and that the conversion of EO was not complete. A combination of three columns, TSKgel G2000 HXL (exclusion limit: 1 × 104) + G3000 HXL (exclusion limit: 6 × 104) + GM HXL (exclusion limit: 4 × 108), which covers the molecular weight range from several hundred to more than 1 × 106, was used for the GPC measurements. No peaks assigned to PEGE homopolymers were detected. 2.3. Determination of the Phase Diagram. The gelation and gel dissolution were determined by a vial inversion method as the temperature was changed between 3 and 90 °C. Glass vials (3 mL) containing aqueous polymer solutions with varying concentrations were kept in a thermostated water bath for more than 1 h prior to their inversion. The state of the polymer solution, which is classified into transparent sol, opaque sol, transparent gel, opaque gel, or syneresis, was distinguished visually, and the gelation temperature was also visually determined when the polymer solutions did not flow by inverting the vials. 2.4. Estimation of Self-Diffusion Coefficients by PGSE-NMR. The PGSE-NMR measurements were conducted by using a JEOL JNM-AL 400 spectrometer with a 9.4 T narrow bore superconducting magnet equipped with a JEOL pulse field gradient probe and a current amplifier providing gradient strengths up to 8.18 T m-1.36 The self-diffusion coefficients were determined using a simple Hahn spin-echo sequence, incorporating a sine gradient pulse in each τ period. The interval between two gradient pulses, ∆, was set at 50 ms, and the duration of the field gradient, δ, was varied in the range between 0 and 1.8 ms. The polymers dissolved in D2O were inserted into a 5 mm (o.d.) NMR microtube, and the D values of the polymers were measured with changing temperature at every 5 °C from 5 to 50 °C by using the 1H signal for the side-chain end methyl group of the PEGE segment (1.2 ppm). 2.5. Estimation of Self-Diffusion Coefficients by DLS. An Ohtsuka Denshi DLS7000 spectrophotometer (a ALV CGS-8F goniometer system and a ALV DLS-5000/ EPP digital correlator) with a Uniphase 22 mW He-Ne laser (λ ) 632.8 nm) as the light (36) Tokuda, H.; Tabata, S.; Susan, M. A. B. H.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2004, 108, 11995-12002.

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Figure 2. Phase diagram of PEGE-b-PEO observed in water. Figure 1. Phase transition temperature of PEGE (Mn ) 3300, 6600, and 8800) in aqueous solution as a function of polymer concentration. source was employed for DLS measurements. The 1 wt % polymer solutions were filtered through a CROMATODISK (0.2 µm pore size) prior to the measurements and equilibrated at each temperature for at least 1 h. The measurements were made with scattering angles of 45, 60, 75, 90, and 110° at temperatures of every 5 °C from 5 to 45 °C. A linear decay rate versus q2 plot was obtained for each decay mode, and the RH values were calculated from the slope of the plots. 2.6. Estimation of Phase Transition Temperature (Tc). The temperature dependence of the transmittance of aqueous PEGE solutions with different concentrations was monitored by a 500 nm light beam through a 1 cm quartz sample cell with a rate of 1 °C min-1 in heating and cooling scans. The Tc value was defined as the temperature at 50% transmittance. The Tc value for the PEGE segment, which corresponds to the critical micellization temperature (cmt) of the block copolymer, was determined according to the following method based on the solubilization of probe molecules within self-assembled polymer aggregates.37 To 4 mL of a 1 wt % aqueous solution of the copolymer, 40 µL of methanol containing 0.4 mmol dm-3 DPH was added, and the obtained mixture was left overnight. The UV-vis absorption spectra of the mixture from 300 to 550 nm were measured at 10, 12, 16, 22, 26, 30, 34, and 39 °C using a Shimazu UV2400PC spectrophotometer. The absorbance at 356 nm was plotted against temperature, and the first inflection of the sigmoidal plot corresponded to the cmt.37 2.7. Observation of Self-Assembly by SAXS. SAXS measurements were carried out by using a Nanoviewer spectrometer (Rigaku Corporation, Japan) equipped with a charge-coupled device (CCD) camera as the detector, for which a rotating generator was operated at 40 kV and 20 mA. The sample was placed in a metal sample holder and sealed by a thin Mylar film.

Figure 3. Absorbance at 356 nm as a function of temperature measured for a 1 wt % PEGE-b-PEO aqueous solution while changing the temperature at 0.5 °C min-1.

3.1. Thermoresponsive Behavior of PEGE. Investigation of the thermoresponsive behavior of PEGE homopolymers helps to understand the behavior of diblock copolymers. Figure 1 shows the Tc values as a function of polymer concentration in water for three PEGE homopolymers with different Mn values. Tc increases with decreasing polymer concentration to below 5 wt %, reflecting the intermolecular aggregation of polymer chains through hydrophobic interaction on the phase transition. 3.2. Phase Diagram. Figure 2 illustrates the phase diagram of PEGE-b-PEO determined by the vial inversion method. Depending on the polymer concentration and temperature, five regions were macroscopically recognized: transparent sol (region I), opaque sol (region II), transparent gel (region III), opaque gel (region IV), and syneresis (region V). The transformation between those phases is reversible with changing temperature. Region I

can be divided into two regions, where copolymer molecules are in the unimer state and in the mixed state of unimers and micelles, respectively. At the boundary between the two regions, the coilglobule transition of the thermoresponsive PEGE segment takes place as shown for the PEGE homopolymers in Figure 1. Figure 3 shows that the absorbance at 356 nm of the copolymer solution containing DPH abruptly increases in the temperature range from 15 to 20 °C. The significant increase in the absorption in the presence of micelles or similar molecular aggregates means that a dye of DPH is transferred from an aqueous environment and solubilized within the interior of the hydrophobic core of aggregates, as schematically shown in Figure 3.38 The temperature-dependent solubilization of DPH and a large surface activity recognized at above 20 °C in an aqueous PEGE-b-PEO solution suggest the formation of micelles from the diblock copolymers. The cmt value is estimated to be 16 °C from the first inflection of the absorption versus temperature sigmoidal curve and is shifted by 5 °C to the higher temperature due to the hydrophilicity of the PEO segment,39 compared with the Tc value for a 1 wt % aqueous solution of PEGE homopolymer having a similar molecular weight. The cmt values are determined mainly by the length of the PEGE segment rather than that of PEO segment.35 Taking into account the less hydrophobic character of the phenyl group (which is less hydrophobic than PT), it is reasonable that the cmt of the present copolymer is more than 5 °C higher than that of PT-PEGE-b-PEO with a similar molecular weight of the PEGE segment. At the boundary between regions I and II, the aggregation of spherical micelles probably takes place and results in macrophase separation into polymer-rich and water-rich domains (section 3.3). The aggregates are only dispersed in water, and they cannot form a gel state up to 90 °C due to the low polymer concentration.

(37) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414-2425.

(38) Grieser, F.; Drummond, C. J. J. Phys. Chem. 1988, 92, 5580-5593. (39) Zhu, P. W.; Napper, D. H. Macromolecules 1999, 32, 2068-2070.

3. Results and Discussion

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Figure 5. Free diffusion signal attenuation versus γ2g2δ2(4∆ δ)/π2 plots measured at various temperatures for 1 wt % PEGEb-PEO in D2O using the 1H signal for the side-chain end methyl group of the PEGE block. Figure 4. SAXS diffraction patterns of 40 wt % PEGE-b-PEO in water as a function of temperature.

A sol-gel transition appears only in the higher concentration region above 40 wt %. The formation of liquid crystalline phases was identified for regions III and IV by observing the samples through cross polarizers. Figure 4 shows the temperaturedependent SAXS spectra measured for an aqueous PEGE solution of 40 wt %. At below 20 °C, only one broad peak was detected, meaning a sol state of the polymer solution. In the temperature range between 20 and 45 °C, three peaks (indicated by the dotted lines 1-3) whose reciprocal spacing is in the ratio of 1:31/2:2 for a hexagonal phase appeared, whereas two peaks (indicated by lines 1 and 2) whose reciprocal spacing is in the ratio of 1:2 for a lamellar phase were observed between 50 and 65 °C. The phase sequence with increasing temperature is the isotropic phase, hexagonal phase, and lamellar phase, which is in agreement with those reported generally.19 It is interesting that the transparent gel (region III) and opaque gel (region IV) phases in Figure 2 seem to correspond to the hexagonal and lamellar liquid crystal phases, respectively. The gelation threshold temperature, which is the boundary between regions I and III, decreases with the concentration of copolymer as shown in Figure 2, reflecting the fact that intermicellar correlations result in the gel phase of hexagonally ordered rods. From region III to region IV, it is probable that dehydration of the PEO chains and the concomitant decrease in the thickness of the hydrophilic shell of hexagonally ordered rods result in the lamellar liquid crystal phase. The opaque character, which reflects the heterogeneity of the gel, suggests the coexistence of the polymer-rich and water-rich domains. With increasing temperature, dehydration of the PEO chains gradually proceeds from the hydrophobic core side to the chain end, and finally the polymer gel excludes water and two phases exist in region V. In contrast to the gelation threshold, the demixing threshold, which is the boundary between regions IV and V, increases with polymer concentration due to the decrease in water content. The following sections focus on the phase transition behavior in 1 wt % dilute aqueous polymer solution. 3.3. Temperature-Dependence of RH of the Block Copolymer Measured by PGSE-NMR. PGSE-NMR has been widely used to determine self-diffusion coefficients of polymers,40-42 (40) Vergara, A.; Paduano, L.; D’Errico, G.; Sartorio, R. PhysChemChemPhys 1999, 1, 4875-4879. (41) Griffiths, P. C.; Stilbs, P.; Yu, G. E.; Booth, C. J. Phys. Chem. 1995, 99, 16752-16756.

surfactants,43 and micelles.44,45 In PGSE-NMR measurements using a sine gradient pulse, the free diffusion echo signal attenuation, E, is related to the experimental parameters by

ln(E) ) ln(S/Sg)0) ) -γ2g2Dδ2(4∆ - δ)/π2

(1)

where S is the spin-echo signal intensity, δ is the duration of the field gradient with the magnitude g, γ is the gyromagnetic ratio, D is the self-diffusion coefficient, and ∆ is the interval between two gradient pulses. Figure 5 shows plots of the logarithm of the attenuation intensity, ln(S/Sg)0), versus γ2g2δ2(4∆ - δ)/ π2 under the present measurement conditions. In the lowtemperature region between 5 and 15 °C, linear plots with relatively steep slopes were obtained. With increasing temperature, the slopes of the plots gradually become small, which corresponds to the decrease in D, and additionally nonlinear plots appear in the middle temperature range from 20 to 35 °C. At temperatures higher than 40 °C, linear plots appear again. The single- and double-exponential fittings were successfully applied to the linear and nonlinear plots, respectively, and the resulting D values were converted to RH values using the StokesEinstein equation: D ) kT/6πRHη, where k is the Boltzmann constant, T is the absolute temperature, and η is the solution viscosity. While Figure 5 is based on the 1H signal for the sidechain end methyl group of the PEGE segment, a similar D value was obtained at each temperature from the 1H signal for the main-chain methylene groups. Figure 6 represents the temperature dependence of the RH value for PEGE-b-PEO. In the temperature region from 5 to 15 °C, the copolymers mainly exist as random coils with RH ) 2-3 nm that are consistent with the RH values of 1.5-3.5 nm for unimers of PEO-PPO-PEO with molecular weights of 280012 500 in aqueous solutions.46,47 This indicates that the copolymer molecules are solvated independently at temperatures below the cmt. Larger polymer nanoparticles with RH values of 10-15 nm start to appear from 20 °C in addition to the unimers with RH ) 2-3 nm, and both smaller and larger nanoparticles coexist in the temperature range from 20 to 35 °C. The RH value of 10-15 nm is also comparable to the size of the micelles for PEOPPO-PEO with similar molecular weights in water.45 (42) Daivis, P. J.; Pinder, D. N.; Callaghan, P. T. Macromolecules 1992, 25, 170-178. (43) Misselyn-Bauduin, A.-M.; Thibaut, A.; Grandjean, J.; Broze, G.; Jerome, R. J. Colloid Interface Sci. 2001, 238, 1-7. (44) Geetha, B.; Mandal, A. B. Langmuir 1995, 11, 1464-1467. (45) Rao, B.; Uemura, Y.; Dyke, L.; Macdonald, P. M. Macromolecules 1995, 28, 531-538. (46) Brown, W.; Schille´n, K.; Hvidt, S. J. Phys. Chem. 1992, 96, 6038-6044. (47) Malmsten, M.; Lindman, B. Macromolecules 1992, 25, 5440-5445.

Preparation and Solution BehaVior of PEGE-b-PEO

Figure 6. Hydrodynamic radius of the copolymer or its aggregates as a function of temperature measured in D2O containing 1 wt % PEGE-b-PEO. The radius was calculated from a self-diffusion coefficient determined by PGSE-NMR using the Stokes-Einstein equation. The inset shows the temperature dependence of the fraction of slow-diffusing components.

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Figure 8. Distribution of the polymer particle radius measured at 10, 25, and 40 °C for a 1 wt % PEGE-b-PEO aqueous solution by DLS.

Figure 9. Hydrodynamic radius of the copolymer or its aggregates as a function of temperature measured for a 1 wt % PEGE-b-PEO aqueous solution by DLS.

Figure 7. 1H NMR spectra of PEGE-b-PEO in D2O at various temperatures.

The 1H signals originate from the side-chain end methyl group of the PEGE segment (1.22 ppm), and the main-chain end phenyl groups (7.0-7.4 ppm) become broader and smaller at above 20 °C, whereas the signal from the main-chain methylene groups (3.7 ppm), that is assigned to both the PEGE and PEO segments, is still less affected up to 40 °C. (Figure 7). This means that the more hydrophobic PEGE segment and the end phenyl group aggregate while the hydrophilic PEO segment is still hydrated at temperatures higher than the cmt, and this is the evidence for the formation of micelles consisting of a PEGE core and a PEO shell. In the temperature range from 20 to 35 °C, the fraction of micelles (nanoparticles with a lower D value in the doubleexponential fitting) gradually increases with increasing temperature (inset of Figure 6) while their size remains constant as shown in Figure 6, suggesting the presence of equilibrium between the micelles and the unimers. At 40 °C, most of the copolymers exist as nanoparticles with RH ) 10-15 nm and a further increase in RH starts. The temperature range, where the second increase in RH occurs, agrees with the phase boundary between the transparent sol and the opaque sol in Figure 2, suggesting that the second increase in RH reflects the aggregation and phase

separation of the core-shell micelles probably triggered by dehydration of the PEO shells. 3.4. Temperature-Dependence of RH of the Block Copolymer Measured by DLS. Figure 8 shows the distribution of the polymer particle radius in 1 wt % PEGE-b-PEO aqueous solution at 10, 25, and 40 °C measured by DLS at a scattering angle of 90°. The scattering intensity largely increased in two steps at around 22 °C and higher than 40 °C, implying that the aggregation of copolymers occurs in these temperature ranges. While only small particles with a radius of ∼1 nm exist at 10 °C, two distribution peaks appear at around 1 and 10 nm at 25 °C. At 30 °C, the average RH values of the smaller and larger particles are 1.8 and 10.4 nm, respectively (data not shown). At 40 °C, the particles with the smaller radius disappear and the remaining distribution peak at ∼10 nm becomes sharp. The RH value of PEGE-b-PEO and its temperature dependence shown in Figure 9 are similar to those determined by PGSA-NMR (Figure 6), although the RH values determined by PGSE-NMR are slightly larger than those determined by DLS. Therefore, the smaller copolymer particles can be assigned to the unimer copolymer coils, and the larger ones can be assigned to the micelles of the copolymers. The size of the micelles generally increases with increasing temperature. However, the decrease in the particle radius with temperature was observed at above the Tc of the PNIPA block for PNIPA-b-PEO.24 On the other hand, the present copolymer exhibits the temperature independence of the particle radius similar to that of PEO-PPO-PEO.48

4. Concluding Remarks We have elucidated the self-assembling behavior of an aqueous solution of PEGE-b-PEO, which was prepared by successive anionic ring-opening polymerization of EGE and EO using 2-phenoxyethanol as a starting substance. We macroscopically identified, with changes in temperature and concentration, five regions in the phase diagram: transparent sol, opaque sol, (48) Nivaggioli, T.; Alexandridis, P.; Hatton, T. A.; Yekta, A.; Winnik, M. A. Langmuir 1995, 11, 730-737.

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transparent gel, opaque gel, and syneresis. For a 1 wt % copolymer solution, the RH value determined by PGSE-NMR and DLS increases with temperature at 15 °C, which corresponds to coreshell micelle formation due to the hydrophilic-hydrophobic transition of the PEGE segment. In the temperature range between 20 and 40 °C, equilibrium between the micelles and the unimers exists, and the fraction of the core-shell micelles gradually increases with increasing temperature while its RH value remains constant. Further aggregation of the core-shell micelles at 40 °C, which is probably triggered by dehydration of the PEO segment, leads to the opaque sol phase. In a 40 wt % solution of the copolymer, dehydration of the PEGE and PEO blocks also plays an important role in the phase transition. SAXS measurements

Ogura et al.

indicate that the transparent gel and opaque gel phases correspond to the hexagonal and lamellar liquid crystalline phases, respectively. Acknowledgment. This work was partly supported by a Grantin-Aid for Scientific Research on Priority Areas of “Chemistry of Coordination Space” (No. 434/17036018, 18033015) and a Grant-in-Aid for Scientific Research (C) (No. 17550125) from MEXT, Japan. We acknowledge Prof. K. Aramaki and the late Prof. H. Kunieda for technical support and helpful discussions in DLS and SAXS measurements. This paper is dedicated to the late Prof. Hironobu Kunieda, Yokohama National University. LA701384Q